Process to manufacture a metal foam provided with channels and metal foam thus produced

ABSTRACT

A manufacturing process for a metal foam provided with at least one channel and intended namely for the manufacture of heat exchangers using a preform of balls. A foundry core constituted by a central core and a coating of a material fusible at low temperature are arranged in a foundry mould, the preform is then tightly arranged around the core, the fusible material is then eliminated by heating at a low temperature, then the molten metal mass is cast in the mould in order to fill the free spaces between the balls and between the balls and the central core, and lastly the balls are eliminated.

The technical scope of the invention is that of the manufacture of metal foams incorporating channels opening on both sides.

These metal foams may be used in particular in heat exchangers to dissipate or circulate heat. However, it is important to have one or several channels that pass from one side to the other of the metal foam so as to produce more complex exchangers enabling the circulation of a second fluid within the exchanger or else to form preferred passages (known as bypasses).

The provision of one or more channels inside a metal foam has already been proposed. Thus, document US 2009/0085520 proposes perforating the foam so as to introduce a tube, either by forcing or by brazing after the introduction of the tube so as to make it integral with the foam. It is understood that this technique requires firstly the manufacture of the metal foam, then its perforation and lastly the installation of a tube integral with the foam. According to this document, the tube is held in place in the foam in various manners, for example by brazing, by compressing the foam around the tube, by the tight fitting of the tube, etc. Lastly, it is understood that this system only enables the insertion of straight tubes.

Patent EP-1808241 discloses a manufacturing process for channels through a metal foam according to which metal tubes are previously inserted into the preform before the molten aluminium or aluminium alloy is cast or else after it is cast. This system provides for the insertion of a preformed tube.

Metal foam technology is well known and reference may be made to patents EP-1808241, U.S. Pat. No. 3,236,706 and EP-2118328 which recommend the manufacture of a preform in the form of salt-based granules or balls, such as sodium chloride. Afterwards, the free space between the granules is filled by a molten metal, the salt being dissolved to recover the metal foam.

Whatever the process used to obtain a channel within the metal foam, the problem of deteriorated conductivity is encountered at the join between the tube and the porous medium of the foam. Moreover, over time, the mechanical strength between the tube and the foam forming two parts is altered. Lastly, it is, to date, impossible to form both the channel and metal foam in a single operation using the same material.

The aim of the invention is to supply a metal foam incorporating a tube which passes through it and is obtained during the manufacture of the foam itself.

The foam is manufactured classically using a preform with granules as described, for example, in the document quoted hereafter.

Indeed, patent EP-2118328 discloses a method that is particularly advantageous by proposing to manufacture a preform using grain flour granules. This preform is thus baked before the metal is cast in order to destroy the carbon chains of the granules. This patent thus firstly provides for the manufacture of a paste formed of flour, sodium chloride and water. The granules to be used thereafter to produce the preform are prepared using this paste.

The invention thus relates to a manufacturing process for a metal foam provided with at least one channel and intended namely for the manufacture of heat exchangers using a preform of balls, characterised in that a foundry core, a central core and a coating of a material fusible at low temperature are arranged in a foundry mould, the preform is then tightly arranged around the core, the fusible material is then eliminated by heating at a low temperature, then the molten metal mass is cast in the mould in order to fill the free spaces between the balls and between the balls and the central core, and lastly the balls are eliminated.

According to a particular embodiment, the channel is constituted, after the metal mass has been cast, by the metal mass replacing the wax coating.

According to another specific embodiment, the central core of the core is formed by ceramic, steel, sand, a soluble material or a material identical to that forming the preform.

According to another embodiment, the core is formed of three elements, a first element constituted by the ceramic, steel or sand core, a second element constituted by a peripheral layer of refractory material and a third element constituted by a coating of material fusible at low temperature.

According to yet another embodiment, the core is formed of four elements, a first element constituted by a ceramic, steel or sand core, a second element constituted by a peripheral layer of a refractory material, a third element constituted by a layer of ceramic material and a fourth element constituted by the coating of material fusible at low temperature.

According to another embodiment, the core is rectilinear.

According to another embodiment, the core has curves.

According to another embodiment, the core is tubular.

According to another embodiment, the low temperature fusible material is a wax.

According to another embodiment, the coating has continuous or discontinuous protrusions on its inner surface.

The invention also relates to the metal foam obtained by following the process according to the invention and provided with at least one channel.

Advantageously, the channel is of a straight or curved tubular shape.

Advantageously again, the channel and foam are aluminium or aluminium-alloy based.

A first advantage of the invention lies in the production of a foam incorporating a channel of the same nature as the foam itself.

An another advantage of the invention lies in the production for the first time and simultaneously of the metal foam and the channel or channels.

Another advantage of the invention lies in the fact of producing a channel of any shape, for example, straight, curved or other.

Yet another advantage of the invention lies in the absence of any negative interaction between the channel and the metal foam.

Yet another advantage of the invention lies in the elimination of any problems of mechanical strength or thermal conductivity between the channel or channels and the metal foam.

Other characteristics, advantages and particulars of the invention will become more apparent from the additional description of the embodiments given hereafter by way of example and with reference to the appended drawings, in which:

FIG. 1 is a view showing a preform in which a core has been inserted,

FIGS. 2-6 show sections views of different embodiments of the core,

FIG. 7 is a section view showing another embodiment of the core, and

FIGS. 8 and 9 are section views of the metal foam with a channel made in situ.

In the following part of the description, the preform shall be considered to have been made in a known manner, which is to say using agglomerated balls made of a salt, such as sodium chloride, and to have been used in a known manner to produce a metal foam. The material constituting the metal foam is also known and, by way of example, aluminium, aluminium alloys and any other known material used in foundry can be used. Reference may be made to the afore-mentioned patent for further details.

As previously mentioned, according to the invention a core is inserted into the preform, such core able to be prepared in different ways. Generally speaking, the invention is based on the replacement of a layer of fusible coating material by the same constituent material as the metal foam so as to produce a channel in situ.

FIG. 1 shows a view of a preform constituted of balls 1, compressed or not, into which a core 2 has been arranged. This core is constituted by a central core 3 and a coating 4. For the sake of clarity, the preform 6 has been shown removed from its mould. This preform is parallelepipedic in shape; the core 2 according to FIG. 1 has been inserted inside it. All the balls 1 can be seen to be in close contact with one another. The extremity of the coating 4 is flush with the side wall of the preform whereas the core 3 is protruding. This illustration is in no way limiting and the invention may just as well be made by allowing the wax to protrude so as to create a skin or to obtain a tube that protrudes with respect to the foam.

Naturally, the core 2 is firstly arranged in the molten metal injection mould in a vertical or inclined position according to the needs of the user and then the balls are poured into the mould. The foam is then produced using known technology which does not require further description here. It goes without saying that it is possible for several cores to be inserted so as to obtain several channels.

FIG. 2 shows a section view of a first embodiment of the core 2 formed of the central core 3 and the coating 4. The central core 3 is constituted, for example, by a ceramic material, steel or sand usually used in foundry technologies. The coating here is constituted by a layer of wax or any other material fusible at low temperatures. A low temperature fusible material is considered to be one whose melting point is of between 40° C. and 150° C. Naturally, the diameter of the central core and the thickness of the layer of coating are determined according to the applications intended for the metal foam. Thus, diameters of the central core 3 of over 2 mm can be imagined and a coating thickness of 0.5 mm to 10 mm (preferably 0.5 to 3 mm) can be adopted.

The Figure shows a rectilinear core, but it goes without saying that a core of any shape may be used. In this case, a central core of sand is preferable.

This preform 1 integrating the core 2 is used classically to manufacture the metal foam. Thus, as the temperature inside the mould rises, the balls are agglomerated so as to create a rigid preform, then the coating 4 is melted and eliminated by simply being poured away. The molten metal is injected in a known manner to take up the spaces between the balls and the free space left by the coating between the preform and the remaining core to constitute the channel. After cooling the central core 3 is extracted or eliminated if made of sand to so to clear the channel formed.

FIG. 3 shows a section view of a variant embodiment of the core 2 made of three parts: the central 3 onto which a layer of refractory material 5 is applied and thereafter a coating 4 of wax or low temperature fusible material. The advantage of this refractory material lies in that it facilitates removal of the core from the mould after the metal has cooled whereas the central core 3 ensures its rigidity. This refractory material also enables the diameter of the final channel to be modified without having to modify the central core.

FIG. 4 shows the core 2 according to FIG. 3 in which the central core 3 has been hollowed out longitudinally to form a channel 6. The advantage of this embodiment lies in the reduction of the material constituting the central core 3. A further advantage of such an embodiment lies in the possibility of being able to circulate an air flow through this channel to improve cooling after casting. The central core is in this case in the shape of a tube which is more or less thick.

FIG. 5 shows a section view of another embodiment of the core 2 made in four parts. The central core 3 is covered by a layer 5 of a refractory material, itself covered with a layer of ceramic material 7 and lastly the coating layer 4. The use of a ceramic material enables the rigidity of the refractory material to be enhanced and any potential infiltration of molten aluminium into the refractory material to be prevented thereby avoiding micro-cracking.

FIG. 6 shows a variant embodiment of that shown in FIG. 5 in which a hollow central core 3 in a tube shape has been provided. This embodiment enables a saving in the constitutive material of the central core.

All the cores previously described with reference to the Figures are inserted into a preform so as to be able to produce one or several channels inside the metal foam. To this end, the core is arranged in the foam production mould and then the balls are put in place and made compact so as to ensure their mutual contact. If necessary, the preform is compressed.

All these cores may incorporate a central core made of a ceramic material, steel or sand, and more generally any material suitable for foundry work. It goes without saying that the central core is made of a ceramic material or of steel if it is in the shape of a straight rod so as to ensure its extraction, the channel obtained being rectilinear. The core may be placed into any position inside the preform and the tube may open out at any point, inlet or outlet, on any face of the metal foam.

FIG. 7 shows another embodiment of the core 2 whose central core 9 incorporates several curves and which is covered by a coating 4 of wax or another low temperature fusible material. In this case, the central core 9 is made of sand or another friable material. The channel obtained inside the foam provides a greater surface area between the fluid circulating through the metal foam and the fluid circulating in the channel.

FIG. 8 shows a section view of a block of metal foam 10 formed by a wire mesh 11 inside which the tubular channel 12 is formed in the free space between the balls and the central core after the coating has been melted. It is understood that the thickness of the channel 12 is substantially equal to the thickness of the wax layer.

The benefit of the present invention is easy to see since the foam 10 and the channel 12 are made simultaneously when the molten metal is cast. They are both thus of the same nature, thereby ensuring the identical conductivity of these two elements and eliminating any problems of mechanical strength.

FIG. 9 shows a longitudinal section view along the channel 12 where channel 12 can be observed to be fully embedded in the mesh 11 of the metal foam with a fusing of the material.

FIGS. 10 and 11 show a variant embodiment consisting in performing grooves or notches on the external surface of the central core 3 (or on the refractory material). Said grooves or notches being filled with coating 4, they cause continuous or discontinuous protrusions 13 along the internal surface of the coating 4 which, when the metal is cast, create exchange surfaces arranged inside the channel so as to improve heat exchange. These protrusions 13 are shown here by way of non-limiting illustration in the form of triangular and rectangular sections. It goes without saying for one skilled in the art that they may be in other forms so as to improve heat exchange whilst preserving the flow of fluid inside the channel. One advantage of this embodiment lies in the increase in the exchange surface inside the channels and thus the performance of the device.

The Figures illustrating the invention show by way of non-limiting example the core, the coating and the tubing with a substantially circular section, it goes without saying that a person skilled in the art will be able to carry out the invention using different sections, namely oval or rectangular.

The foams according to the invention are particularly suited to the production of heat exchangers, of whatever type, liquid/liquid, or liquid/gas, or gas/gas or phase-changing fluids (liquid−>gas). 

1. A manufacturing process for a metal foam provided with at least one channel and intended namely for the manufacture of heat exchangers using a preform of balls, wherein a core constituted by a central core and a coating of a material fusible at low temperature are arranged in a foundry mould, the preform then tightly arranged around the core, the fusible material is then eliminated by heating at a low temperature, then the molten metal mass is cast in the mould in order to fill the free spaces between the balls and between the balls and the central core, and lastly the balls are eliminated.
 2. A process according to claim 1, wherein the channel is constituted, after the metal mass has been cast, by the metal mass replacing the wax coating.
 3. A process according to claim 1, wherein the central core or the core is formed by ceramic, steel, sand, a soluble material or a material identical to that forming the preform.
 4. A process according to claim 1, wherein the core is formed of three elements, a first element constituted by the ceramic, steel or sand central core, a second element constituted by a peripheral layer of refractory material and a third element constituted by a coating of material fusible at low temperature.
 5. A process according to claim 1, wherein the core is formed of four elements, a first element constituted by a ceramic, steel or sand central core, a second element constituted by a peripheral layer of a refractory material, a third element constituted by a layer of ceramic material and a fourth element constituted by the coating of material fusible at low temperature.
 6. A process according to claim 1, wherein the central core is rectilinear.
 7. A process according to claim 1, wherein the central core has curves.
 8. A process according to claim 1, wherein the central core is tubular.
 9. A process according to claim 1, wherein the low temperature fusible material is a wax.
 10. A process according to claim 1, wherein the coating has continuous or discontinuous protrusions on its inner surface.
 11. A metal foam obtained by following the process according to claim 1 and provided with at least one channel.
 12. The metal foam according to claim 11, wherein the channel is in the form of a rectilinear or curved tube.
 13. The metal foam according to claim 11, wherein the channel and foam are made of aluminium or aluminium alloy. 